1. Field of the Invention
The present invention concerns a device for the injection of a plastic material, especially a thermosetting plastic material, into a mold. It concerns, notably, the coating of electronic components, such as capacitors.
2. Description of the Prior Art
Electronic components such as capacitors are generally coated with a plastic material designed to insulate and protect them from the effect of phenomena that could harm their operation, namely phenomena such as humidity, aggressive chemical agents, mechanical or electrical shocks, etc. It is preferred to coat electronic components with a thermosetting plastic material, which has the advantage after it is used, of being infusible and insoluble.
Thermosetting materials are materials that get converted, by chemical reaction, into three-dimensional macro-molecular compounds. Their use requires the effect of heat. Unlike thermoplastic materials, the chemical conversion process which has converted a thermosetting material into a thermoset material is irreversible. The non-melting quality of the thermoset plastic materials is of great importance for electronic components.
There are different known methods of molding. According to the compression molding method, the material to be molded is placed in an open hot mold which is then closed by means of a hydraulic press. Under the effect of heat and pressure imposed by the press, the material fluidizes and takes the shape of the mold. This method has some drawbacks: equipment is immobilized for long periods, the molding cycle is fairly long and burrs have to be removed.
In the transfer molding method, the material to be molded goes from a transfer pot or cavity, where the material is compressed and fluidized, into a hot mold that is kept firmly closed. The material is transferred to the mold by means of channels machined in the mold. The pressure exerted on the material to make it move forward in the channels of the mold contributes to its heating through frictional phenomena. This method gives a shorter baking time than the compression molding method, but this time is still too long for the fabrication of electronic components at a high rate. It calls for the application of greater pressure.
In the injection molding method, the thermosetting material is first fluidized by heating and then sent into a mold. This method enables total automation of the molding chain. There is neither manipulation nor loss of time between the end of plastification and the injection. It is by this method that the shortest baking times are obtained. The drawbacks of this method are the high degree of wear and tear of the screw and the relatively large amount of wastes due to the lengths of the sprues remaining in the mold.
In the application to electronic components, it is the injection molding method that appears to be the most promising one. It enables manufacture at a high rate according to an entirely automated molding cycle. However, the above-mentioned drawbacks are such that transfer molding continues to be widely used in employing a polyepoxy resin in powder form.
In order to enable total automation of a capacitor fabrication line, the present applicant has been led to develop an original device enabling the injection of a thermosetting plastic designed to fill capacitor packs. For this, it was necessary to find satisfactory approaches to enable molding under very low injection pressures (0.2 to 1 MPa), very short periods (30 to 40 seconds) of polymerization at low temperatures (about 100.degree. C.) and a fairly long pot life (10 to 12 hours) of the hardener-resin mixture.
The device according to the invention enables the molding of liquid formulae, hence without pelleting and pre-heating operation. It enables the molding of parts that are very brittle mechanically (through the low injection pressure) and thermally (owing to the low temperature of the mold used).
The device according to the invention meets yet another requirement which is that of preventing all contact with the liquid resin which gives rise to allergies in users. For, through its design, the injection device remains clean after use, and the cleaning operations are thereby simplified.
The device according to the invention does not use any injection screw which has the drawback of getting quickly worn out.
Although the invention has been developed for a particular application concerning capacitors, the injection device may be used in other fields by those skilled in the art.